Our previous work showed that certain weak acids, especially aspirin and salicylate, induce nonheritable resistance to ampicillin, chloramphenicol, nalidixic acid and tetracycline in Escherichia coli. Similar results have now been obtained in various other bacteria including certain Bacillus, Klebsiella, Salmonella, Shigella and Vibrio, but not Streptococcus or Staphylococcus species. Furthermore, acetaminophen (Tylenol) has also been found to induce noninheritable drug resistance E. coli. Several additional effects of salicylate on E. coli have been demonstrated. In the presence of salicylate, the bacteria require methionine or isoleucine and valine, appear to be Lac-, and are hypersensitive to kanamycin. Mutants that do not show these traits are being isolated and characterized.